researchC

Jäkle Group
Research in Organometallic & Polymer Chemistry

Research

Supported by:


Project C. Metallocene-based bidentate Lewis acids

Bidentate Lewis acids are highly desirable reagents with applications, for example, as catalysts in asymmetric Diels-Alder and Aldol reactions in organic synthesis, as cocatalyst in olefin polymerization; they also serve as selective chemosensor materials. In our lab, we develop new routes to tailor-made bi- and multidentate Lewis acids, in which multiple Lewis acid centers can interact in a cooperative fashion, thus leading to enhanced substrate binding. We are particularly interested in metallocene-based multifunctional Lewis acids due to their unusual geometric features and favorable electrochemical properties.[1]
Recently, we have reported the first examples of ferrocene-based planar-chiral bidentate Lewis acids such as 1.[2] Through X-ray crystallography and multinuclear NMR studies, we have confirmed highly unusual cooperative binding effects between the Lewis acidic boron and tin centers, leading to novel boron-chiral pyridine complexes (2) and reverse chelates with fluoride.[3,4] These planar chiral bidentate Lewis acids are promising candidates for chiral synthesis, with the stereoselective allylation of ketones being a first example.[5]
X-ray structure courtesy of Prof. Rheingold
	Another class of ferrocene-based bidentate Lewis acids that we are studying are biferrocenes containing two boron centers embedded into a highly rigid redox-active framework as encountered in 1,2-[fcB(Cl)]2 (3a).[6] We have discovered that the related highly rigid biferrocene species 1,2-[fcB(Ph)]2 (3b) upon partial oxidation shows unusually strong electronic coupling between the ferrocene groups,which is indicative of fast electron transfer through the novel dibora-s-indacene ligand framework.[7,8] Reversible reduction on the other hand leads to a delocalized radical anion. Interestingly, conformational changes are triggered either through redox chemistry or through binding of nucleophiles, thus rendering 3b an interesting candidate for the design of new types of molecular machines (see C&EN 09/05/2005 issue). The oxidized diboradiferrocene is a promising candidate for sensor applications, as oxidation leads to strong Lewis acidity enhancement.[9]
We have also prepared a series of reverse chelates with the dimercury bidentate Lewis acid 1,2-fc(HgCl)2.[10] And finally a highly unusual 1,2-dicuproferrocene complex is formed with copper, an aggregate that consists of 4 ferrocene and 8 pentafluorophenylcopper moieties. Most intriguingly, the iron centers of the ferrocenes appear to interact simultaneously with multiple copper Lewis acid sites.[11]

References:

(1) F. Jäkle, in Group 13 Chemistry: Fundamental Research, Materials Science and Catalysis, Ed. D. Atwood, P. J. Shapiro, ACS Symposium Series No. 822, 2002, 104-111.

(2) J. A. Gamboa, A. Sundararaman, L. Kakalis, A. J. Lough, F. Jäkle, Organometallics 2002, 21, 4169-4181; "Ferrocene-Based Heteronuclear Bidentate Lewis Acids via Highly Selective ortho-Borylation of 1,1’-Bis(trimethylstannyl)ferrocene".

(3) R. Boshra, A. Sundararaman, L. N. Zakharov, C. D. Incarvito, A. L. Rheingold, F. Jäkle, Chem. Eur. J. 2005, 11, 2810-2824; "Binding Cooperativity of Two Different Lewis Acid Groups at the Edge of Ferrocene".

(4) R. Boshra, K. Venkatasubbaiah, A. Doshi, R. A. Lalancette, L. Kakalis, F. Jäkle, Inorg. Chem. 2007, 46, 10174-10186; "Simultaneous Fluoride Binding to Ferrocene-Based Heteronuclear Bidentate Lewis Acids".

(5) R. Boshra, A. Doshi, F. Jäkle, Angew. Chem Int. Ed. 2008, in press; "Allylation of Ketones with a Ferrocene-Based Planar Chiral Lewis Acid".

(6) K. Venkatasubbaiah, T. Pakkirisamy, A. Doshi, R. A. Lalancette, F. Jäkle, Dalton Trans. 2008, submitted; "Tuning the Electronic Structure of Diboradiferrocenes".

(7) K. Venkatasubbaiah, L. N. Zakharov, W. S. Kassel, A. L. Rheingold, F. Jäkle, Angew. Chem. Int. Ed. 2005, 44, 5428-5433; "Reversible Expansion and Contraction of a 1,2-Diborylated Ferrocene Dimer Promoted by Redox Chemistry and Nucleophile Binding".

(8) K. Venkatasubbaiah, A. Doshi, I. Nowik, R. H. Herber, A. L. Rheingold, F. Jäkle, Chem. Eur. J. 2008, in press; "Examination of the Mixed Valence State of the Doubly Boron-bridged Diferrocene Cation [(FeCp)2(mu-C10H6(BPh)2)]+"; (highlighted with frontispiece).

(9) K. Venkatasubbaiah, I. Nowik, R.H. Herber, F. Jäkle, Chem. Commun. 2007, 2154-2156; "Lewis Acidity Enhancement of Organoboranes via Oxidation of Appended Ferrocene Moieties".

(10) K. Venkatasubbaiah, J. W. Bats, A. L. Rheingold, F. Jäkle, Organometallics 2005, 24, 6043-6050;
"Rational Synthesis and Complexation Behavior of the Bidentate Lewis Acid 1,2-Bis(chloromercury)ferrocene".

(11) K . Venkatasubbaiah, A. G. DiPasquale, A. L. Rheingold, M. Bolte, F. Jäkle, Angew. Chem. Int. Ed. 2006, 45 , 6838-6841; "Mono- and Dimetalated Ferrocenylcopper Complexes via Tin-Copper Exchange ".

Research

Project A

Project B

Project D